1. Field of the Invention
The present invention relates to a surface mounting type small size antenna that is used in the communication field, particularly used by being incorporated in mobile communication equipment such as cellular phones and, in particular, relates to a small size surface mounting antenna and an antenna device which deal with double channels as well as a radio communication device using the same.
2. Description of the Related Art
In recent years, miniaturization of mobile communication equipment such as cellular phones has made rapid progress. Together with the miniaturization of such communication equipment, antennas which are incorporated in communication equipment are also required to be miniaturized and a small size antenna where a conductor is formed on the surface of a base in rectangular parallelepiped form so that the antenna is mounted on the surface of an internal board of communication equipment has been put into practical use.
FIG. 4 is a perspective view schematically showing a small size antenna according to the prior art and FIG. 5 is a cross sectional view in the longitudinal direction of FIG. 4. As shown in FIG. 4, in a small size antenna 15 which is mounted on surfaces of a board 4 within communication equipment by means of solders 3 according to the prior art, conductors 2 are formed on the surface of a base 13 in rectangular parallelepiped form and are electrically connected to wires (not shown) on the board 4. In general, the conductors 2 are formed over two or more surfaces of the base 13 and, for example, the conductors 2 are sequentially formed over four surfaces that include surfaces 8 and 11 as shown in FIG. 5. The conductors 2 are formed by screen printing a conductor paste on each surface in a sequential manner.
As for the method for electrically connecting the conductors 2 on the adjoining two surfaces to each other as described above, in general, a screen having a pattern greater than the pattern of a conductor 2 is utilized, thereby, generating a sag on the next surface so that such a sag is utilized for an electrical connection. FIG. 6 is an enlarged cross sectional view showing a small size antenna according to the prior art where a screen printing has been carried out by using a screen having a pattern greater than the pattern of the conductor 2 on the surface 8. As shown in FIG. 6, the conductor 2 printed on the surface 8 causes the sag 18 on the surface 11. This sag 18 makes contact with the conductor 2 on the surface 11 to form an electric connection when the conductor 2 is screen printed on the surface 11.
In the case where such a small size antenna is mounted on the surface of a board in a cellular phone or the like, this small size antenna is secured to the board 4 by means of the solders 3 as shown in FIG. 4. It becomes necessary to scrap the board 4 together with the antenna, though this is economically disadvantageous, in the case where the properties of the antenna are found to be defective as a result of quality management where the frequency properties are measured after the antenna has been secured to the board 4 by means of the solders 3. Here, in general, the frequency is measured by using a network analyzer in order to inspect the antenna to see if it functions properly.
Therefore, the small size antenna 15 is installed on the board 4 without being secured by means of the solders 3 so that the frequency is measured in this condition or the small size antenna 15 is secured onto the board 4 by applying pressure to the small size antenna 15 from above or by sucking the antenna from beneath so that the frequency is measured.
However, in a small size antenna according to the prior, as shown in FIG. 6, the conductor 2 is printed by using a screen having a pattern greater than that of the conductor 2 and, thereby, a protrusion 23 is also formed simultaneously as the sag 18 is formed. This is considered to occur because some conductor paste stays in the portions of the screen as a result of the usage of the screen having a pattern greater than that of the conductor 2.
Here, in the case where the frequency of the small size antenna 15 on which the protrusion 23 has been formed is attempted to be measured, no problem with the measurement arises when the antenna is secured onto the board 4 by using the solder 3 as shown in FIG. 8A because the solder 3 fills in the space between the conductor 2 and the board 4 in spite of the existence of the protrusion 23. However, in the case where measurement is carried out without utilizing the solder 3, as shown in FIG. 8B, a gap occurs between the board 4 and the conductor 2 due to the protrusion 23 and a problem arises where the frequency becomes different from that of the case where measurement is carried out when the antenna is secured to the board by using the solder 3.
It is considered that this is because air, which is a dielectric body, exists in the space between the board 4 and the conductor 2, which occurs due to the protrusion 23, and, thereby, a difference is made in the electrical properties leading to a difference in the frequency.
In addition, in the case where the base 13 in rectangular parallelepiped is utilized, a problem arises where chipping occurs when bases 13 collide into each other during the manufacturing process for bases 13 in rectangular parallelepiped form or during the process for forming conductors 2 or base 13 collides into a jig or the like that is utilized during the manufacturing process. In the case where chipping has occurred, the product might become defective judging from the appearance and at the same time, in the case where chipping has occurred to a portion of the conductor 2, the product loses the function as an antenna due to the occurrence of disconnection.
In order to avoid this problem, there is a method for preventing chipping of ceramic where a C surface or a step is provided on an outer periphery portion. FIG. 7 is an enlarged cross sectional view schematically showing a small size antenna according to the prior art in the case where a step 21 has been created in a corner portion of the base 13 when the antenna is formed using dies for powder press formation.
As shown in FIG. 7, a step 21 made of an inclining portion 19 and a flat portion 20 is provided in a corner portion of base 13 and, thereby, the occurrence of chipping of the base 13 is prevented. In the case of a small size antenna, however, an electrical connection between the conductors 2 on the surface 8 and on the surface 11 is necessary while it becomes necessary for length g of the flat portion 20 to be 0.08 mm or greater taking the strength of dies for an extended period of time into consideration in the case where the step 21 is created in the corner portion of the base 13 by means of the dies. A problem arises where some products lack the properties required for an antenna as a result of the occurrence of an electrically insufficient connection when the connection is made using a sag of the conductor 2 in the above described space of 0.08 mm or longer.
In addition, in recent years a surface mounting type antenna that deals with double channels and an antenna device using the same has been introduced to the market and dynamic development thereof has progressed. In the following, an antenna that deals with two frequencies according to the prior art is described in reference to the perspective view of FIG. 13.
In FIG. 13, a surface mounting type antenna, which is denoted as 81, is mounted on a mounting substrate 92 so as to form an antenna device. In the surface mounting type antenna 81, shown in FIG. 13, a base in rectangular parallelepiped form is denoted as 86, a feeding terminal is denoted as 85 and radiation electrodes are denoted as 82 and 83. In addition, in the mounting substrate 92, a feeding electrode is denoted as 94 and a grounding conductive layer is denoted as 93.
In the surface mounting type antenna 81 of this FIG. 13, two frequencies can be dealt with by changing the pitches of the radiation electrodes 82 and 83. That is to say, the pitch of the radiation electrode 83 in spiral form that is connected to the feeding terminal 85 on a side of the base 86 is expanded while the pitch of radiation electrode 82 in spiral form that is connected to the radiation electrode 83 is condensed and, thereby, the antenna is formed so as to be able to deal with two different frequencies.
Thus, the surface mounting type antenna 81 that has been formed as described above is mounted on the surface of the mounting substrate 92 and the feeding terminal 85 is connected to the feeding electrode 94, thereby forming the antenna device 91 that deals with two frequencies.
In addition, an antenna for a mobile communication terminal is disclosed as another type of antenna that deals with two frequencies in a manner wherein a ground capacitance of an antenna element is connected to an antenna element for a predetermined frequency band so as to change this value and, thereby, the antenna can be utilized in a plurality of frequency bands that includes another frequency band different from the predetermined frequency band (see for example, Japanese Unexamined Patent Publication No. 2002-204120). This antenna makes it unnecessary to insert a switch in series in a transmission path of signals that are transmitted and are received and, therefore, it can be said that it is possible to form an antenna which can cope with a plurality of frequencies without causing a problem of signal transmission loss.
In addition, an antenna device is also disclosed wherein a dielectric base, a plurality of power supply radiation elements each of which has a feeding electrode and a radiation electrode formed on the surface of this base, and a substrate for securing the base are provided where one power supply point for supplying power to all of the power supply radiation elements is provided on this substrate, and at the same time, stubs are provided on the surface of the substrate or on the surfaces of the base and the substrate so as to be sequentially deployed starting from the power supply point so that the feeding electrode of a power supply radiation element is connected to the matching point of a stub that is determined based on the effective line length of the radiation electrode (see for example, Japanese Unexamined Patent Publication No. 2002-314330). This antenna device allows each power supply radiation element to be excited by the resonant frequency that is determined by the effective line length of a radiation electrode and at this time, the feeding electrode of each power supply radiation element is connected to the matching point of the stub having the optimal stub length for each power supply radiation element and, therefore, each power supply radiation element can gain a resonance properties which are appropriate in each of the resonant frequencies. It can be said that such an antenna device allows a necessary band width of the frequency band to which each of the resonant frequencies belongs to be secured.
In the surface mounting type antenna 81 according to the prior art as shown in FIG. 13, however, it is necessary to adjust the lengths and the pitches (intervals) of the radiation electrodes 82 and 83 in spiral form in order to tune the operation frequency of the surface mounting type antenna 81 to each of low frequency f1 and high frequency f2 of the radio signals utilized in the communication system and a problem arises where a large amount of time and effort is required for such adjustments.
In addition, when the surface mounting type antenna 81 is attempted to be miniaturized by increasing the dielectric constant of the base 86, an unnecessary resonance mode unexpectedly occurs between the long radiation electrodes 82, 83 in spiral forms and the grounding conductor 93 preventing stable antenna properties that deal with two frequencies from being gained and, thus, a problem arises where the miniaturization of the antenna is difficult.
In addition, there is a problem in the antenna for a mobile communication terminal that is disclosed in Japanese Unexamined Patent Publication No. 2002-204120 where it is difficult to mount the antenna on the surface of a mounting substrate.
Furthermore, there is a problem in the antenna device disclosed in Japanese Unexamined Patent Publication No. 2002-314330 where it is difficult to miniaturize the antenna because the radiation electrodes have two-dimensional patterns increasing the size of the antenna